Conventional cardiac pacemakers and defibrillators consist of a generator for an electricity source and an elongated flexible pacemaker lead that is connected proximally to a header structure on the generator and is implanted distally within the heart for cardiac pacing and defibrillation. A lead is in actuality a bare segment of an insulated wire. The cardiac lead is configured with a tubular electrically insulated sleeve structure that is inserted into the body through an incision overlying veins leading to the heart chambers where the distal end of lead is lodged. The distal end of the lead is connected to a tubular tip electrode, having an increased diameter forming an annular shoulder against which the distal end of the sleeve abuts.
Biocompatible silicone based adhesives are used to connect the distal end of the lead sleeve and the tip electrode. Among the limitations of adhesives is that the manufacturing of the assembled lead requires sufficient time for the adhesive to cure, and the adhesive's bond strength may decrease in time and permit separation of the tip electrode from the sleeve.
One or more ring-type electrodes are secured to the sleeve proximal to the tip electrode. A flexible stylet is used to maneuver the lead to a desired position in cardiac chambers or veins with the guidance of real time imaging fluoroscopy. This is where cardiac stimulation occurs by contact with cardiac tissue.
Two conventional anchoring mechanisms are typically employed for fixing the distal end of a lead to cardiac tissue. An active fixation mechanism usually involves a screw-in electrode tip. A passive fixation mechanism consists of one or more radial tines that engage the inner lining of the heart or blood vessels.
Two conventional stimulation devices are typically employed, a single chamber device and dual chamber device. A single chamber system is capable of sensing and pacing in one chamber, either in the atrium or in the ventricle. Dual chamber stimulation devices are capable of sensing and pacing in both chambers, that is, in both the atrium and ventricle. Modes of pacing include VDD, DVI, VVI, and DDD, where the first letter indicates the chamber being paced, and the second letter indicates the chamber being sensed, while the third letter indicates inhibited or triggered responses. A fourth letter “R” denotes rate responsive pacing to match a patient's activities. In addition to pacing the right atrium and right ventricle, pacing the left ventricle via the cardiac veins, or biventricular pacing, provides more physiologic and synchronous cardiac contraction, and may improve cardiac function.
A lead system consists of one or more leads, conductor coils, and electrodes. The conductor coil is the internal core of the lead though which electric current flows. The lead is an insulated wire that connects the stimulator to the cardiac tissue. The lead carries impulses to the tissues one the one hand, and cardiac signals back to the sensing circuitry of the stimulator or generator on the other.
There are basically two types of leads, unipolar and bipolar. A unipolar lead has one conductor coil, with typically a cathode, or negative pole, at the distal tip and an anode, or positive pole, defined by the housing of the stimulator. The electric current returns to the anode via body tissue as a current path. A bipolar lead has two conductor coils, the distal tip forming a cathode, and an annular or ring electrode located a few millimeters proximal to the distal tip. For purposes of delivering high voltage defibrillation, one or two shocking coils are inserted intravenously.
Pacemaker leads are generally suited for placement in the ventricle and atrium. In order to provide permanent pacing and to avoid pacemaker lead dislodgment, various methods have been use to anchor the leads to the endocardium (the inner lining of the heart chambers). There has been increasing evidence in the literature that conventional right ventricular apical pacing alters the normal synchronization of different heart chambers, and may adversely influence ventricular function, leading to heart failure (inability of the heart to pump the required volume of blood) and increased mortality. Mark H. Schoenfeld reviewed the literature and pointed out the potential adverse consequences of right ventricular apical pacing (Circulation 2007; 115:638-653).
Alternative pacing sites from the His bundle, right ventricular outflow pacing, coronary sinus and cardiac veins have been utilized. Biventricular pacing or resynchronization requires the placement of electrodes inside the venous system of the heart. However, other than lodging the tip of the lead into the distal coronary vein, there is no safe anchoring mechanism to keep the lead from dislodging. Moreover, the best lodging site may not be the ideal pacing location for effective myocardial stimulation. Thus screw-in anchor may apply to the myocardium, but cannot be utilized in vascular structures due to the risk of endothelial damage and hemorrhage.
Another disadvantage of conventional pacemaker right ventricular leads is that they must cross the tricuspid valve. For that reason, the leads may cause unwanted tricuspid regurgitation by interfering with tricuspid valve closing during heart contraction. This may interfere with right ventricular function.